Following the guideline her life depends on, a diver threads the needle through a stalagmite forest in Dan’s Cave on Abaco Island. A single, misplaced fin kick can shatter mineral formations tens of thousands of years old.

Bacteria color the water at a depth of 30 to 36 feet in Sawmill Sink on Abaco. Here and in a colorless layer below, poisonous hydrogen sulfide gas is present. Divers move through it with all deliberate speed.

“All of a sudden, it’s got you,” says photographer Wes Skiles of the “insanely dangerous” vortex in Chimney Blue Hole off Grand Bahama. Like a giant bathtub drain, it sucks down millions of gallons when the tide comes in. “It’s like going over a waterfall—there’s no escape.” Keeping his distance, a diver sets up equipment to measure the whirlpool’s flow rate.

Resting casually on a ledge 80 feet down, free diver William Trubridge admires the yawning entrance of Dean’s Blue Hole on Long Island. Trubridge holds the world record for a breath-holding free dive—a three-minute, 56-second odyssey in this Bahamian cave, to a depth of 311 feet.

Kenny Broad sinks into the toxic hydrogen sulfide layer in Sawmill Sink on Abaco Island. By studying bacteria that flourish in the oxygen-free water, scientists hope, among other things, to learn how simple life-forms can lead to more complex ones.

In Garbage Hole on Grand Bahama, Kenny Broad squirms through a narrow section, spooling out a guideline for a safe return to the surface. Although tidal currents sweep litter into this offshore cave’s deepest reaches, its walls teem with life, including bright red bryozoans; smooth, gray sponges; and bushy, stinging hydroids that can raise welts on exposed skin. For many cave divers, entering a previously unexplored passage—as Broad is doing here—is the holy grail.

The Cascade Room, some 80 feet beneath the surface, leads divers deeper into Dan’s Cave on Abaco Island. Nearly seven miles of the cave have been explored since the mid-1990s.Panorama composed of three images

Through a halo of dive lights, Kenny Broad ascends a deep shaft in Dan’s Cave on Abaco. The site is one of the world’s most spectacular inland underwater caves, thanks to its abundant mineral formations, from columns and curtains to calcite “soda straws” that can break at the touch of a fingertip.

Amid a hanging forest of stalactites in Ralph’s Cave on Abaco, Brian Kakuk shines his dive light on a translucent stalagmite. During periods of lower sea level, when caves were dry, stalagmites and stalactites grew and eventually joined to form columns.

His air bubbles forced down by the current in a blue hole on Abaco, Kenny Broad fights to the surface, a stalagmite under his arm. Divers must bring extra breathing gas when they’ll have to struggle against a siphoning tide.

Tires and other debris have piled up in Garbage Hole on Grand Bahama. “You’re swimming through people’s drinking water and junk piles at the same time,” Kenny Broad says. “It really drives home why we need to protect these places.”

A diver’s-eye view just below the surface reveals the jungled entrance of Ben’s Cave in Lucayan National Park on Grand Bahama. Established in 1970, the park protects one of the longest inland underwater cave systems on Earth.